The use of air-to-air heat pumps is extremely widespread in the field of comfort air conditioning. Heat pumps of this type are used to cool and dehumidify in summer and to heat in winter on the basis of a conventional vapor compression cycle. The shift between cooling and heating modes is effected by reversing the direction of refrigerant flow and correspondingly interchanging the roles of condenser and evaporator in the cycle.
Heat pumps of the type described hereinabove have a number of significant limitations. Firstly, the comfort levels provided thereby sometimes fall significantly below desired levels. Secondly a relatively high level of electricity consumption is required in view of the comfort level provided.
These limitations have a number of aspects. In humid summer weather, it is normally not economically feasible to maintain the conditioned volume at a relative humidity of less than approximately 50%. Conventional apparatus does not provide further dehumidification which would increase the comfort level for the same room temperature or alternatively allow a reduction in electricity consumption by permitting an increase in the room temperature while maintaining the same comfort level.
Concomitantly, in winter weather conventional heat pump apparatus does not provide humidification which would increase the comfort level for the same room temperature or alternatively allow a reduction in electricity consumption by permitting a decrease in the room temperature while maintaining the same comfort level.
It is known to provide apparatus separate from the heat pump for humidifying or dehumidifying. Such apparatus is normally less efficient that the heat pump itself, requires a separate water connection and involves periodic maintenance.
An additional difficulty arises from frosting of the evaporator coils of conventional heat pumps in the winter. This may occur whenever moisture condenses on the coil at freezing temperatures. Conventional heat pumps which are provided with defrosting apparatus must disable their normal functioning during the operation of the defrosting apparatus and require additional energy for the defrost function.
It may be appreciated that the operation of conventional heat pumps in the summer and the winter involves the wastage of a moisture transfer potential due to the significant difference between the relative humidity of the air streams exiting at the evaporator and the condensers.
U.S. Pat. No. 4,180,985 describes a method and apparatus for summer cooling and dehumidification wherein a vapor compression refrigeration system is equipped with a regeneratable dessicant for contacting moist feed air prior to passing the feed air across the evaporator coils of the system. The dessicant removes moisture from the feed air thereby improving the efficiency of the air conditioning system. The dessicant material is regenerated by utilizing waste heat which is removed from the condenser of the air conditioning system.
The teaching of U.S. Pat. No. 4,180,985 involves a number of difficulties. Firstly, the adsorption capacity of the dessicant is relatively low due to the limited amount of regeneration which can be provided by the relatively low temperature of the air stream at the condenser output. Thus, the relative humidity of the air supplied to the air conditioned room by the apparatus may not be significantly less than that produced by conventional apparatus not incorporating the dessicant, thereby involving the drawbacks already discussed above.
Concomitantly, the apparatus of U.S. Pat. No. 4,180,985 cannot be used for simultaneous heating and humidifying because the cool feed air entering the dessicant is of too high a relative humidity to pick up any appreciable moisture.
No solution is proposed to the problem of frosting.
U.S. Pat. No. 4,259,849 describes a chemical dehumidification system which utilizes a refrigeration unit for supplying energy to the system and in which air passes first through a dehumidifier unit prior to passing an evaporator. A corresponding heating system is not provided.
U.S. Pat. No. 2,946,201 proposes the use of a regeneratable dessicant to dehumidify freezer room air in order to avoid frosting of cooling coils.
U.S. Pat. No. 2,138,689 illustrates the use of a dessicant for humidification.